The present technique relates generally to the field of electrical machines, and particularly to rotors for induction devices, such as a fabricated squirrel cage rotor, for example.
Electrical machines, such a motors and generators, are commonly found in industrial, commercial, and consumer settings. In industry, such machines are employed to drive various kinds of devices, including pumps, conveyors, compressors, fans, and so forth, to mention only a few, and are employed to generate electrical power. In the case of electric motors and generators, these devices generally include a stator, comprising a multiplicity of stator windings, surrounding a rotor.
By establishing an electromagnetic relationship between the rotor and the stator, electrical energy can be converted into kinetic energy, and vice-versa. In the case of alternating current (ac) motors, ac power applied to the stator windings effectuates rotation of the rotor. The speed of this rotation is typically a function of the frequency of the ac input power (i.e., frequency) and of the motor design (i.e., the number of poles defined by the stator windings). Advantageously, a rotor shaft extending through the motor housing takes advantage of this produced rotation, translating the rotor's movement into a driving force for a given piece of machinery. Conversely, in the case of an ac generator, rotation of an appropriately magnetized rotor induces current within the stator windings, in turn producing electrical power.
Often, design parameters call for relatively high rotor rotation rates, i.e., high rpm. By way of example, a rotor within a high-speed induction motor may operate at rates as high as 10,000 rpm, and beyond. Based on the diameter of the rotor, operation at such rpm translates into relatively high surface speeds on the rotor. Again, by way of example, these rotor surface speeds can reach values of 100 meters per second (mps), and beyond. During operation, particularly during high-speed operation, produced centripetal and centrifugal forces strain various components of the rotor assembly. For example, if not properly accounted for, the centripetal and centrifugal forces developed in the end ring may cause the end ring to prematurely malfunction. Moreover, these centripetal and centrifugal forces may, overtime, negatively affect the mechanical integrity of the rotor, leading to a lessening of performance and, in certain instances, failure of the motor. Undeniably, loss of performance and motor failure are events that can lead to unwanted costs and delays.
In traditional motors, the end ring and the electrical conductors extending through the rotor core are mechanically and electrically coupled via a brazing process (e.g., solder). By way of example, the conductor and the end ring may be soldered together using a hard solder with a high melting point. Unfortunately, heat generated during a brazing process can negatively affect the material of the end rings and/or the conductor. For example, developed heat can cause annealing in the end rings and/or conductors. In turn, such annealing reduces the yield strength of the annealed material, thereby increasing the likelihood of damage due to centripetal and centrifugal forces within the rotor, for instance.
A technique for mechanically securing the end rings to the rotor core is described in U.S. patent application Ser. No. 10/955,680 entitled “HIGH MECHANICAL STRENGTH ELECTRICAL CONNECTION SYSTEM AND METHOD,” which was filed on Sep. 30, 2004, and incorporated herein by reference. By way of example, the end rings may be mechanically secured to rotor through the use of a copper bushing that surrounds ends of the rotor's conductor bars and are located within end slots of the end rings into which these conductor bars extend. In summary, the abutment of the bushing with the rotor's conductor bars and the respective end rings presents a mechanical engagement that secures the end rings to the rotor and, furthermore, secures the rotor assembly.
When employing such mechanical techniques, difficulties can arise during the assembly process. For example, it can be burdensome to transfer the rotor core stack between a station for stack compression and a station bushing insertion. Moreover, such bifurcated assembly employs two independent machines, increasing costs for manufacturing. Additionally, providing for symmetrical insertion of the bushings often present challenges that, in some cases, can lead to asymmetries in the assembled rotor. Such asymmetries can negatively impact rotor performance.
There exists a need, therefore, for a method and apparatus for improved rotor construction and integrity.